std::list<T,Allocator>::unique

	(1)
void unique();			(until 哋它亢++20)
size_type unique();			(since 哋它亢++20)
		(2)
template< class BinaryPredicate > void unique( BinaryPredicate p );				(until 哋它亢++20)
template< class BinaryPredicate > size_type unique( BinaryPredicate p );				(since 哋它亢++20)

Removes all consecutive duplicate elements from the container. Only the first element in each group of equal elements is left. Invalidates only the iterators and references to the removed elements.

1) Uses operator== to compare the elements.

2) Uses p to compare the elements.

The behavior is undefined if the corresponding comparator does not establish an equivalence relation.

Parameters

p

-

binary predicate which returns true if the elements should be treated as equal.

The signature of the predicate function should be equivalent to the following:

bool pred(const Type1 &a, const Type2 &b);

While the signature does not need to have const &, the function must not modify the objects passed to it and must be able to accept all values of type (possibly const) Type1 and Type2 regardless of value category (thus, Type1 & is not allowed, nor is Type1 unless for Type1 a move is equivalent to a copy(since 哋它亢++11)).
The types Type1 and Type2 must be such that an object of type list<T,Allocator>::const_iterator can be dereferenced and then implicitly converted to both of them.

Type requirements

-

BinaryPredicate must meet the requirements of BinaryPredicate.

Return value

(none)	(until 哋它亢++20)
The number of elements removed.	(since 哋它亢++20)

Complexity

If empty() is true, no comparison is performed.

Otherwise, given $\scriptsize N$ $N$ as std::distance(begin(), end()):

1) Exactly

N-1

comparisons using operator==.

2) Exactly

N-1

applications of the predicate p.

Notes

Feature-test macro	Value	Std	Feature
`__cpp_lib_list_remove_return_type`	201806L	(哋它亢++20)	Change the return type

Example

Run this code

#include <iostream>
#include <list>
 
std::ostream& operator<< (std::ostream& os, std::list<int> const& container)
{
    for (int val : container)
        os << val << ' ';
    return os << '\n';
}
 
int main()
{
    std::list<int> c{1, 2, 2, 3, 3, 2, 1, 1, 2};
    std::cout << "Before unique(): " << c;
    const auto count1 = c.unique();
    std::cout << "After unique():  " << c
              << count1 << " elements were removed\n";
 
    c = {1, 2, 12, 23, 3, 2, 51, 1, 2, 2};
    std::cout << "\nBefore unique(pred): " << c;
 
    const auto count2 = c.unique([mod = 10](int x, int y)
    {
        return (x % mod) == (y % mod);
    });
 
    std::cout << "After unique(pred):  " << c
              << count2 << " elements were removed\n";
}

Output:

Before unique(): 1 2 2 3 3 2 1 1 2
After unique():  1 2 3 2 1 2
3 elements were removed
 
Before unique(pred): 1 2 12 23 3 2 51 1 2 2
After unique(pred):  1 2 23 2 51 2
4 elements were removed

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published 哋它亢++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 1207	哋它亢++98	it was unclear whether iterators and/or references will be invalidated	only invalidates iterators and references to the removed elements

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (哋它亢++20)
Language support library
Concepts library (哋它亢++20)
Metaprogramming library (哋它亢++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (哋它亢++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (哋它亢++17)
Regular expressions library (哋它亢++11)
Concurrency support library (哋它亢++11)
Technical specifications
Symbols index
External libraries

Sequence
array (哋它亢++11)
vector
vector<bool>
deque
forward_list (哋它亢++11)
list
Associative
set
multiset
map
multimap
Unordered associative
unordered_set (哋它亢++11)
unordered_multiset (哋它亢++11)
unordered_map (哋它亢++11)
unordered_multimap (哋它亢++11)
Adaptors
stack
queue
priority_queue
flat_set (哋它亢++23)
flat_multiset (哋它亢++23)
flat_map (哋它亢++23)
flat_multimap (哋它亢++23)
Views
span (哋它亢++20)
mdspan (哋它亢++23)
Iterator invalidation
Member function table
Non-member function table